There is a great need in industries such as the semiconductor industry for sensitive metrology equipment that can provide high resolution and non-contact evaluation capabilities, particularly as the geometries of devices in these industries continue to shrink. Manufacturers have increasingly turned to optical metrology techniques, such as ellipsometry and reflectometry, which typically operate by illuminating a sample with a probe beam of electromagnetic radiation and then detecting and analyzing the reflected and/or transmitted energy. The probe beam can consist of polarized or unpolarized radiation, and can include one or more wavelengths of radiation in any of the appropriate radiation bands as known in the art. Ellipsometry techniques typically measure changes in the polarization state of the probe beam after interacting with the sample, while reflectometry techniques measure changes in the magnitude of the reflected probe beam. Scatterometry is a specific type of optical metrology that typically is used to measure diffraction, or optical scattering, of the probe beam due to the structural geometry of the sample, whereby details of the structure causing the diffraction can be determined.
Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (BBS), described in U.S. Pat. Nos. 5,607,800, 5,867,276, and 5,963,329; spectral ellipsometry (SE), described in U.S. Pat. No. 5,739,909; single-wavelength optical scattering (SWOS), described in U.S. Pat. No. 5,889,593; and spectral and single-wavelength beam profile reflectance (BPR) and beam profile ellipsometry (BPE), described in U.S. Pat. No. 6,429,943. Scatterometry in these cases generally refers to the collection of optical response information in the form of diffraction orders produced by periodic structures, such as gratings on a wafer. Any of these measurement technologies, such as single-wavelength laser BPR or BPE technologies, also can be used to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas. The above cited patents and patent applications, as well as PCT Application No. WO 03/009063, U.S. application Ser. No. 2002/0158193, U.S. application Ser. No. 2003/0147086, U.S. application Ser. No. 2001/0051856 A1, PCT Application No. WO 01/55669 and PCT Application No. WO 01/97280, are each hereby incorporated herein by reference.
For optical metrology systems utilizing broadband light, the outputs of two or more light sources or bulbs are often combined in order to obtain a probe beam with suitable broadband characteristics. For example, three lamps can be used to generate a probe beam that spans a wavelength range from about 185 nm to about 900 nm. A tungsten lamp is often used due to the associated output range from the visible to near infrared spectrum, a deuterium bulb is often used for the associated deep ultraviolet (DUV) output, and a xenon bulb is often used for the associated deep ultraviolet to near infrared output spectrum.
One problem with using light sources such as those listed above is that each light source can generate undesirable thermal radiation outside the intended wavelength band. This undesirable radiation can contaminate the desired in-band signals at the detector, such that some error is introduced into the measurements. In order to minimize the amount of undesirable thermal radiation, a heat-blocking filter can be used at any appropriate point between the light source and the detector. For instance, a heat-blocking filter can be used that blocks all undesirable out-of-band infrared light, without blocking desirable in-band signals over the wavelength range of ultraviolet to near-infrared light.
Many existing systems have utilized various heat-blocking filters. For example, a laser notch filter that transmits over a narrow pass band and blocks infrared light can be used in a laser system. Other common filters include a band pass filter that transmits over the visible spectrum and blocks infrared light, as well as filter glass that transmits in the visible spectrum and cuts off in the infrared spectrum. None of these existing filters is entirely satisfactory for all applications, as these filters cannot provide for high transmission and/or high reflection over a large wavelength range, such as a range spanning 185 nm to 900 nm, which also blocks or removes infrared heat beyond 900 nm.